[roll] Roll fuchsia [starnix][vdso] Simplify and speed up UTC clock computation
Read both utc_now and the backstop on memory mapped clock read.
This allows us to determine both `now` and whether the clock
is started, with minimal work.
Technically GetDetails does that, but it also does way more than
needed. Since this is a very short but very hot path, that extra
throwaway work caused perf regressions.
Before: 37ns/44ns
After: 37ns/22ns
Before, raw data:
```
{
"name": "BM_VDSOClockGettime/1",
"family_index": 1, "per_family_instance_index": 0,
"run_name": "BM_VDSOClockGettime/1", "run_type": "iteration", "repetitions": 1,
"repetition_index": 0,
"threads": 1, "iterations": 18630107,
"real_time": 3.7491921007215083e+01,
"cpu_time": 3.7489518015113816e+01,
"time_unit": "ns"
}, {
"name": "BM_VDSOClockGettime/0",
"family_index": 1,
"per_family_instance_index": 1, "run_name": "BM_VDSOClockGettime/0",
"run_type": "iteration",
"repetitions": 1,
"repetition_index": 0,
"threads": 1,
"iterations": 15841102,
"real_time": 4.4255490306166877e+01,
"cpu_time": 4.4253978100766261e+01,
"time_unit": "ns"
}
```
After, raw data:
[stdout - fuchsia-pkg://fuchsia.com/starnix_gvisor_clock_gettime_benchmark?hash=d756a5cd483b311f67dfe1abf6b13d6d9bad6909de9b83e228665312e70c927d#meta/clock_gettime_benchmark.cm]
},
{
"name": "BM_VDSOClockGettime/1",
"family_index": 1,
"per_family_instance_index": 0,
"run_name": "BM_VDSOClockGettime/1",
"run_type": "iteration",
"repetitions": 1,
"repetition_index": 0,
"threads": 1,
"iterations": 18789472,
"real_time": 3.7154366019467972e+01,
"cpu_time": 3.7153508145412488e+01,
"time_unit": "ns"
},
{
"name": "BM_VDSOClockGettime/0",
"family_index": 1,
"per_family_instance_index": 1,
"run_name": "BM_VDSOClockGettime/0",
"run_type": "iteration",
"repetitions": 1,
"repetition_index": 0,
"threads": 1,
"iterations": 31697677,
"real_time": 2.2225429327202555e+01,
"cpu_time": 2.2223562313414959e+01,
"time_unit": "ns"
}
]
}
Original-Bug: 456248727
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1415700
Original-Revision: 1b5d5f445bb509728fe320f3638cfb2bd1d6c905
GitOrigin-RevId: d362b6afbdfe73eaec5ecc9823c144419b331d62
Change-Id: I22981ccc8940a598ea8279d4d947791008b0a289
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